A massive disk/envelope in shocked H₂ emission around an UCHII region

¹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 7150-462 Porto, Portugal
² Instituto Superior da Maia, Av. Carlos Oliveira Campos, 4475-690 Avioso S.Pedro, Portugal
³ Harvard Smithsonian Center for Astrophysics, 60 Garden Street, MS-78 Cambridge, MA 02138, USA
⁴ Joint Astronomy Center, 660 N. A'ohōkū Place, University Park, Hilo, HI 96720, USA
⁵ Instituto de Astronomia, UNAM-Morelia, Apartado postal 3-72, CP 58090 Morelia, Michoacan, Mexico

Corresponding author: M. S. N. Kumar, nanda@astro.up.pt

Received: 23 April 2003
Accepted: 2 September 2003

Abstract

A multi-wavelength study of IRAS 07427–2400 in line and continuum emission was conducted to investigate the nature of a H₂ –0 S(1) line emitting feature around this ultra-compact HII region. High resolution 3.6 cm continuum observations from the Very Large Array and 350 μm continuum observations from the Caltech Submillimeter Observatory, combined with archival far-infrared data of IRAS 07427–2400 show a flux density distribution indicating a luminous () point source associated with an ultra-compact HII region. A Grey body model fit to the flux density distribution yields a dust emissivity index () indicative of a circumstellar disk/envelope. Our C¹⁸O map shows a dense core centered on the continuum source, with the major axis roughly aligned with the H₂ feature. A position-velocity diagram of the C¹⁸O core obtained along the major axis shows rotation with a velocity gradient of ~0.1 km s^-1 arcsec^-1. New CO –2 maps of the region are presented which reveal a massive molecular outflow from the IRAS source. We argue that the H₂ feature arises in a disk/envelope around IRAS 07427–2400 and not in an outflow. We present a near-infrared HK band spectrum of the H₂ features that shows several ro-vibrational emission lines of H₂ and [FeII]. Analysis of the line ratios indicates that the line emission is shock-excited and not due to fluorescence. We estimate an excitation temperature of ~1600 K and an average extinction of  mag to the H₂ feature. The line fluxes yield a mass accretion rate of  yr^-1 and a lifetime of ~53601200 yr resulting in a disk/envelope mass of 14050 . The resulting Jeans Mass of 2420  indicates that the disk/envelope will not undergo fragmentation. IRAS 07427–2400 represents one of the most massive YSOs known to date forming by means of accretion.